Electrical switching devices are known in various embodiments, commonly known by the term "relay". Electromagnetic relays have been available for years, but they demand a lot of space, energy, and they generate electrical noise when the contacts make and break. Such devices also require a relatively high control power, and are thus precluded from use in a number of tasks, e.g. where the controlling power is being provided by a computer.
Other kinds of electrical switching circuits are based only upon electronics, i.e. making and breaking is being performed without mechanical contacts. Semiconductor technology is utilized for this purpose. These so-called "SSR-relays" ("Solid State Relay") have great heat losses with high loads, especially with inductive loads. They thus need to be cooled and for this reason they are precluded for a number of tasks. In particular they are precluded for use over a long period of time.
U.S. Pat. No. 4,074,333 (Murakami, at al.) discloses a device in which these detrimental features are eliminated. Said device operates by means of first energizing the load using an electronic coupling means and a bidirectionally controlled, contactless switch, whereupon a mechanical relay connects and holds the load circuit and the contactless switch opens. The order of making and breaking is controlled by a dedicated sequence controller. Means is provided for controlling the triac (contactless switch), responsive to signals from the sequence controller through a phase detector. The phase detector is provided to ensure making and breaking at the point in time where the phase angle in the load circuit equals zero (zero-voltage crossing). A signal is fed back from the triac-controller to the sequence controller, which, through the energizing means, provide closing of the electromagnetic relay.
An advantage of the device disclosed in U.S. Pat. No. 4,074,333 compared to directly using an electromechanical relay, lies in the fact that arcs are avoided in making and breaking the load circuit. The said load circuit is first energized by the contactless switch. This implies utilizing the advantages from both kinds of switches, the "SSR"-technology provides a non-arc making, and the electromagnetic relay provides a permanent connection without substantial heat losses.
A disadvantage with the device disclosed in U.S. Pat. No. 4,074,333 is that it comprises a relatively complex circuit including a plurality of circuit complex circuit elements. If this circuit should be designed according to the description, using existing circuit elements, it would become unreasonably expensive. Furthermore, the device would require a relatively great amount of space, so that the device would have to be large and expensive, and thus of less commercial interest.